Network with several subnetworks

ABSTRACT

The invention relates to a network with several subnetworks, which are organized either decentrally or centrally and can be connected in each case each by bridge terminals, a proxy terminal for a bridge terminal being set up in at least one of the subnetworks, which proxy terminal during an absence (dictated by frequency, time, code, or other factors) of the bridge terminal accepts all data directed to the bridge terminal or to be forwarded thereby, temporarily stores the data, and forwards said data to the bridge terminal when this is present again.

[0001] The invention relates to a network with several subnetworks, eachconnectable by bridge terminals.

[0002] The subnetworks may operate at different frequencies or codes orat different times. Within each individual network, terminalscommunicate in a wireless manner over one or more radio sections.Furthermore, a central monitoring station may or may not be presentwithin a subnetwork.

[0003] Such a network is known for example from “Habetha, J.; Nadler,M.: Concept of a Centralised Multihop Ad Hoc Network; ProceedingsEuropean Wireless, Dresden, September 2000”. In this known network,adjoining subnetworks operate at different frequencies and are linkedtogether by bridge terminals that are in the overlap range of twosubnetworks. The bridge terminals take part alternately in the operationwithin the two subnetworks, by switching back and forth from onefrequency to the other. There is the possibility in each of thesubnetworks of suppressing any transmission to the bridge terminal whileit is operating at the frequency of another subnetwork. This is managedby a central monitoring station in each of the subnetworks, which isresponsible for the assignment of transmission resources within thesubnetwork and is informed about the absence of the bridge terminal.During the absence of the bridge terminal, the central monitoringstation thus allocates no transmission capacity to any other stationswhich have applied for a transmission directed at the bridge terminal.

[0004] It is an object of the invention to provide a network withimproved communication possibilities between the subnetworks. The objectis reached according to the invention with a network with severalnetworks, each connectable by bridge terminals, wherein it is providedthat in at least one of the subnetworks a proxy terminal for a bridgeterminal is set up, which during an absence of the bridge terminalaccepts data directed to the bridge terminal or to be forwarded by thebridge terminal, temporarily stores the data, and forwards said data tothe bridge terminal when this is present again.

[0005] According to the invention, a bridge terminal therefore chooses adifferent station as its proxy, which accepts and temporarily stores alldata directed at the bridge terminal during the absence of the bridgeterminal. When the bridge terminal switches back to the frequency (thecode or time range) of the subnetwork under consideration, the proxythen passes to the bridge terminal all the data accepted on its behalfin the preceding period.

[0006] The present invention relates advantageously to a network inwhich in at least one subnetwork either no central monitoring stationexists or for some other reason a transmission to the bridge terminalcannot be suppressed during its absence.

[0007] The object is also reached according to the invention with abridge terminal according to claim 5 and a proxy terminal according toclaim 6.

[0008] The networks may be organized either decentrally or centrally.

[0009] The absence may be dictated by frequency, time, code or otherfactors.

[0010] If a central monitoring station exists in a subnetwork, but atransmission to the bridge terminal is not possible during its absence,the central monitoring station is chosen by the bridge terminal as itsproxy, according to a preferred embodiment of the invention.

[0011] The invention will be described in more detail below withreference to embodiments shown in the drawings, in which:

[0012]FIG. 1 shows an ad hoc network with three subnetworks, eachcontaining terminals provided for radio transmission, FIG. 2 shows aterminal of the local network as in FIG. 1, FIG. 3 shows a radio deviceof the terminal as in FIG. 2, and FIG. 4 shows an implementation of abridge terminal provided for connecting two subnetworks, and the proxyterminal of this bridge terminal.

[0013] The embodiment presented in the following relates to ad hocnetworks, which in contrast to traditional networks are self-organizing.Every terminal in such an ad hoc network can enable access to a fixednetwork and can be used immediately. An ad hoc network is characterizedin that the structure and the number of participants are not fixedwithin predefined limiting values. For example, a communication deviceof a participant can be taken out of the network or linked in. Incontrast to traditional mobile telecommunication networks, an ad hocnetwork is not dependent on a permanently installed infrastructure.

[0014] The size of the ad hoc network's area is generally very muchgreater than the transmission range of a terminal. A communicationbetween two terminals can therefore necessitate the activation offurther terminals, so that these can transfer messages or data betweenthe two communicating terminals. Such ad hoc networks, in whichforwarding of messages and data via a terminal is necessary, arereferred to as multihop ad hoc networks. Multihop ad hoc networks caneither be operated at a frequency (or a code or time range), or consistof sub-networks that each operate at a different frequency, code or timerang. A subnetwork of the ad hoc network may be formed, for example, byterminals, connected over radio links, of participants sitting round atable. Such terminals may be, for example, communication devices forwireless exchange of documents, images, etc.

[0015] Two types of ad hoc networks can be specified. These aredecentralized and centralized ad hoc networks. In a decentralized ad hocnetwork, the communication between the terminals is decentralized, i.e.each terminal can communicate directly with any other, provided that theterminal is within the other terminal's transmission range in each case.The advantage of a decentralized ad hoc network is its simplicity androbustness against errors. In a centralized ad hoc network certainfunctions, such as the function of multiple access of a terminal to theradio transmission medium (Medium Access Control=MAC), are controlled byone particular terminal per subnetwork. This terminal is referred to asthe central terminal or central controller (Central Controller=CC).These functions need not always be executed by the same terminal: theycan be transferred from one terminal serving as the central controllerto another terminal then acting as central controller. The advantage ofa central ad hoc network is that an agreement on the quality of service(QoS) is easily possible in it. One example of a centralized ad hocnetwork is a network that is organized according to the HIPERLAN/2 HomeEnvironment Extension (HEE) (see J. Habetha, A.Hettich, J. Peetz, Y. Du,“Central Controller Handover Procedure for ETSI-BRAN HIPERLAN/2 Ad HocNetworks and Clustering with Quality of Service Guarantees”, 1^(st) IEEEAnnual Workshop on Mobile Ad Hoc Networking & Computing, Aug. 11, 2000).

[0016]FIG. 1 shows an embodiment of an ad hoc network with threesubnetworks 1 to 3, which each contain several terminals 4 to 16.Constituents of the subnetwork 1 are the terminals 4 to 9, of thesubnetwork 2 the terminals 4 and 10 to 12, and of subnetwork 3 theterminals 5 and 13 to 16. In a subnetwork, the terminals belonging tothat subnetwork exchange data over radio links. The ellipses drawn inFIG. 1 specify the radio coverage area of a subnetwork (1 to 3), inwhich a largely problem-free radio transmission is possible between theterminals belonging to the subnetwork.

[0017] The terminals 4 and 5 are called bridge terminals, because theseenable data exchange between two subnetworks 1 and 2 or 1 and 3respectively. The bridge terminal 4 is responsible for the data trafficbetween the subnetworks 1 and 2, and the bridge terminal 5 for the datatraffic between the subnetworks 1 and 3.

[0018] A terminal 4 to 16 of the local network according to FIG. 1 canbe a mobile or a fixed communication device and contains, for example,at least one station 17, a connection-checking device 18, and a radiodevice 19 with an antenna 20, as shown in FIG. 2. A station 17 may be,for example, a portable computer, telephone, etc.

[0019] As is shown in FIG. 3, a radio device 19 of the terminals 6 to 16comprises besides the antenna 20 a radio-frequency circuit 21, a modem22, and a protocol device 23. The protocol device 23 forms packet unitsfrom the data stream received from the connection-checking device 18. Apacket unit contains part of the data stream and additional controlinformation formed by the protocol device 23. The protocol device usesprotocols for the LLC layer (LLC=Logical Link Control) and the MAC layer(MAC=Medium Access Control). The MAC layer controls the multiple accessof a terminal to the radio transmission medium, and the LLC layerperforms a flow and error control.

[0020] As was mentioned above, a specific terminal may be responsiblefor the monitoring and management functions in a subnetwork 1 to 3 of acentralized ad hoc network, and in this case is referred to as a centralcontroller. The controller also works as a normal terminal in theassociated subnetwork. The controller is responsible, for example, forthe registration of terminals that start operations in the subnetwork,for the connection setup between at least two terminals in the radiotransmission medium, for the resource management, and for the accesscontrol in the radio transmission medium. Thus, for example, afterregistering and after signaling a desire to transmit, a terminal in asubnetwork is allocated transmission capacity for data (packet units) bythe controller.

[0021] In the ad hoc network, the data may be exchanged between theterminals by a TDMA, FDMA, CDMA, or CSMA method (TDMA=Time DivisionMultiplex Access, FDMA=Frequency Division Multiplex Access, CDMA=CodeDivision Multiplex Access, CSMA=Carrier Sense Multiple Access). Themethods may also be combined. Each subnetwork 1 to 3 of the localnetwork is assigned a number of specific channels, referred to aschannel groups. A channel is defined by a frequency range, a time rangeor, for example with the CDMA method, by a spreading code. For example,a specific frequency range, different in each case, with a carrierfrequency f_(i) may be available to each subnetwork 1 to 3 for dataexchange. In such a frequency range, data can be transferred by TDMA orCSMA, for example. The carrier frequency f₁, may be allocated to thesubnetwork 1, the carrier frequency f₂ to the subnetwork 2 and thecarrier frequency f₃ to the subnetwork 3. The bridge terminal 4 operateson the one hand at the carrier frequency fi in order to be able toexecute a data exchange with the other terminals of subnetwork 1, and onthe other hand at the carrier frequency f₂ in order to be able toexecute a data exchange with the other terminals of subnetwork 2. Thesecond bridge terminal 5 included in the local network, which transfersdata between the subnetworks 1 and 3, operates at the carrierfrequencies f₁, and f₃.

[0022]FIG. 4 is a block diagram of an embodiment of a bridge terminal.The construction of the proxy terminal may also be executed in the sameway. The radio switching device for this bridge terminal comprises aprotocol device 24, a modem 25 and a radio-frequency circuit 26 withantenna 27. The protocol device 24 is connected to a radio switchingdevice 28, which is further connected to a connection-checking device 29and a buffer memory device 30. The buffer memory device 30 in thisembodiment comprises a storage element, is used for temporary storage ofdata, and is implemented as a FIFO module (First In First Out), i.e. thedata is read from the buffer memory device 30 in the order in which itwas written into it. The existence of a buffer memory facility for theproxy terminal is of special significance, since this temporarily storesall data directed to the bridge terminal during its absence. The memorycan be divided into logic areas for separate storage of the data fromdifferent connections. The terminal shown in FIG. 4 can likewise work asa normal terminal. Stations connected to the connection-checking device29, which are not drawn in FIG. 4, then deliver data via theconnection-checking device 29 to the radio switching device 28.

[0023] The bridge terminal of FIG. 4 is synchronized alternately with afirst and a second subnetwork. Synchronization is understood to be theentire process of integrating a terminal up to the exchange of data. Ifthe bridge terminal is synchronized with the first subnetwork, it canexchange data with the terminals adjacent to it in radio range, and witha controller (if present) of this first subnetwork. If data is deliveredfrom the connection-checking device 29 to the radio switching device 28,its destination being a terminal or the controller of the firstsubnetwork or a terminal or controller of another subnetwork which canbe reached via the first subnetwork, the radio switching device forwardsthis data directly to the protocol device 24. The data is temporarilystored in the protocol device 24 until the time slot determined by thecontroller for the transmission is reached. If the data output from theconnection-checking device 29 is to be sent to a terminal or thecontroller of the second subnetwork or to another subnetwork accessiblevia the second subnetwork, the radio transmission must be delayed untilthe time slot in which the bridge terminal is synchronized with thesecond subnetwork. The radio switching device therefore routes the datawhose destination is in the second subnetwork or accessible via thesecond subnetwork, to the buffer memory device 30, which temporarilystores the data until the bridge terminal is synchronized with thesecond subnetwork.

[0024] If data from a terminal or the controller of the first subnetworkis received by the bridge terminal, its destination being a terminal orthe controller of the second subnetwork or a terminal or controller ofanother subnetwork accessible via the second subnetwork, this data islikewise stored in the buffer memory device 30 up to the synchronizationwith the second subnetwork. Data whose destination is a station of thebridge terminal is passed directly via the radio switching device 28 tothe connection-checking device 29, which then routes the received datato the desired station. Data whose destination is neither a station ofthe bridge terminal nor a terminal or controller of the secondsubnetwork is sent, for example, to a further bridge terminal.

[0025] After the change of synchronization of the bridge terminal fromthe first to the second subnetwork, the data stored in the buffer memorydevice 30 is read back from the buffer memory device 30 in the order inwhich it was written. Then, during the period of time that the bridgeterminal is synchronized with the second subnetwork, all data whosedestination is a terminal or the controller of the second subnetwork, oranother subnetwork accessible via the second subnetwork, can be passedon at once from the radio switching device 28 to the protocol device 24,and only the data whose destination is a terminal or the controller ofthe first subnetwork, or another subnetwork accessible via the firstsubnetwork, is stored in the buffer memory device 30.

[0026] The proxy terminal is constructed similarly to the bridgeterminal, but does not perform a frequency change. It is informed aboutits proxy role in a once-only or periodic explicit signal from thebridge terminal. The proxy terminal can refuse this role. If it acceptsthe proxy role, the bridge terminal informs the proxy terminal about thestarting time and duration of its absence (or once only about theperiods of its presences and absences).

[0027] If during the absence of the bridge terminal data is receivedfrom a terminal or a controller of the first subnetwork, its destinationbeing the bridge terminal or-a terminal or controller of the secondsubnetwork accessible via the bridge terminal, this data is stored inthe buffer memory device 30 of the proxy terminal until the return ofthe bridge terminal to the first subnetwork. Data whose destination is astation of the proxy terminal itself is passed directly via the radioswitching device 28 to the connection-checking device 29, which thenroutes the received data to the desired station.

[0028] After the return of the bridge terminal, the data stored in thebuffer memory device 30 of the proxy terminal is read back from thebuffer memory device 30 in the order in which it was written and sent tothe bridge terminal. The proxy terminal may either be informedexplicitly by a signal from the bridge terminal about its return, or itmay infer the return time implicitly from the periods of the presenceand absence times for the bridge terminal. Afterwards, during the periodof time that the bridge terminal is synchronized with the firstsubnetwork, all data whose destination is a terminal or a controller ofthe second subnetwork, or another subnetwork accessible via the secondsubnetwork, can be accepted by the bridge terminal itself.

[0029] As an example of a possible embodiment for two adjoiningsubnetworks, one of the subnetworks could work in accordance with theIEEE Standard 802.11, while the second subnetwork to be connected wouldoperate in accordance with the ETSI Standard HIPERLAN/2. This wouldpresuppose that the bridge terminal was able to communicate inaccordance with both standards. In this case a proxy terminal only hasto be set up in the first (CSMA-based) subnetwork, which is working inaccordance with the IEEE 802.11. In the second HIPERLAN/2 subnetwork,the so-called central controller could suppress all transfers to thebridge terminal during its absence. As was noted for the general case,according to the invention, the central monitoring station of the 802.11network, called the “Point Coordinator (PC)” or “Hybrid Coordinator(HC)”, should (if active) be selected as the proxy terminal in the802.11 based subnetwork. If no PC/HC is active in the first subnetwork,the bridge terminal may choose any terminal in this network as the proxyterminal. For example, in this case the best receivable adjoiningterminal of this network could be selected as proxy terminal. Theinvention is thus suitable for connecting together networks operating inaccordance with different standards.

[0030] Another embodiment of the invention could, for example, involvethe linking together of two or more subnetworks of the same standard. Iftwo adjoining subnetworks operate work in accordance with the IEEE802.11 Standard at different frequencies, for example, at least onebridge terminal and a proxy terminal should be set up in each of the twosubnetworks.

[0031] Finally, a further possible arrangement of the proxy conceptshould be described in which at least two or more bridge terminals areset up between the same subnetworks. If the bridge terminals coordinatetheir presence in the adjoining subnetworks in such a way that always atleast one bridge terminal is present (this is suggested, for example, in“Peetz, J.: HiperLAN2 Multihop Ad Hoc Communication byMultiple-Frequency Forwarding, Vehicular Technology Conference, Rhodes,May 2001”), the bridge terminal present any given time the time couldact as the proxy terminal for the other bridge terminals set up betweenthe same subnetworks.

1. A network with several subnetworks, each connectable by bridgeterminals, wherein it is provided that in at least one of thesubnetworks a proxy terminal for a bridge terminal is set up, whichduring an absence of the bridge terminal accepts data directed to thebridge terminal or to be forwarded by the bridge terminal, temporarilystores the data, and forwards said data to the bridge terminal when thisis present again.
 2. A network as claimed in claim 1, characterized inthat, if a central monitoring station exists in the subnetworkconcerned, the central monitoring station should be selected as theproxy terminal for the bridge terminal.
 3. A network as claimed in claim1, characterized in that at least two subnetworks operate in accordancewith different communication standards, and the bridge terminals areprovided to be used and to be able to operate in accordance with bothcommunication standards for data exchange of those subnetworks thatoperate in accordance with different communication standards.
 4. Anetwork as claimed in claim 1, characterized in that, if several bridgeterminals exist between the same subnetworks, another bridge terminal isselected as the proxy terminal.
 5. A bridge terminal, which is used inseveral subnetworks of a network for connecting the subnetworks andwhich selects a proxy terminal that during the absence of the bridgeterminal receives all data directed to the bridge terminal, temporarilystores the data, and forwards said data to the bridge terminal when thisis present again.
 6. A proxy terminal which during the absence of aterminal receives all data directed to the terminal, temporarily storessaid data, and forwards said data to the bridge terminal when this ispresent again.